Steel composition, particularly for anti-friction bearings

ABSTRACT

A steel composition having improved wear resistance and shaping properties required particularly for antifriction bearings comprising admixtures reducing the final eutectoid carbon content of the alloy.

United States Patent [191 Habrovec et al. 1 Jan. 7, 1975 I STEEL COMPOSITION, PARTICULARLY FOR ANTI-FRICTION BEARINGS [52] U.S. Cl. 148/36, 75/126 0 [75] Inventors: Framisek Habrovec; Jiri Skarek; [51] Int. Cl. C220 29/00 Premysl RyS, an of Frantisek [58] Field of Search 148/36; 75/126 0 Mrkva, Zbysov u Rosic; Jiri Zeman, Brno, all of Czechoslovakia; [56] References Cited MII'OSlaV Janik, deceased, late Of Brno, Czechoslovakia by Drahomire 7 Janik wife and heiress Zdenek ,354,l47 7/1944 Scott 75/126 O K B C h l k. 2,844,500 7/1958 PCHIS v v 148/30 ines, m0, ZEC O5 Ova la 3,663,314 5/1972 Manama 75/1260 [73] Assignee: Ceskoslovenska Akademie Ved, 3963.315 5/1972 K111919612 9 148/36 Pram Czechoslovakia 3,726,724 4/1973 Davies 148/36 [22] Filed: June 1973 Primary Examinerl-lyland Bizot [21] Appl. No.: 365,953

Related U.S. Application Data ABSTRACT [63] Continuation-in-part of Ser. Nos, 883,578, Dec. 9, A steel composition having improved wear resistance 1969, abandoned, and Set Dec. and shaping properties required particularly for anti- 1970- friction bearings comprising admixtures reducing the final eutectoid carbon content of the alloy. [30] Foreign Application Priority Data Dec. 9, 1968 Czechoslovakia 8368-68 2 Claims, N0 Drawings STEEL COMPOSITION, PARTICULARLY FOR ANTI-FRICTION BEARINGS This application is a continuation-impart application of Ser. No. 883,578, filed on Dec. 9, 1969, now abandoned, and application Ser. No. 98,440, filed coassigned 15, 1970, both co-signed to the same assignee.

This invention relates to a steel composition for antifriction bearings and for machine parts which are subject to intensive wear, which steel composition shows properties improving the quality and operating life of bearings and which reduces production costs of bearings.

Parts of anti-friction bearings are actually mostly manufactured from overeutectoid chromium steel of an approximate chemical composition of 1 percent carbon, 0.3 percent manganese, 1.5 percent chromium, remainder substantially iron. This steel, after quenching and low-temperature tempering, has a hardness 61 64 H strength limit 150 200 kp/mm proportionality limit 90 130 kplmm and elongation 0.5 1 percent. The high hardness obtained by the heat processing is advantageous for the function of rolling bearings since the components of the bearings are stressed especially in compression and the compression stresses attain, in accordance with Hertz values, up to 500 kp/mm The high hardness also permits the maintenance of a high dimensional accuracy particularly during the proper operation of anti-friction bearings made from the alloy. But the shaping properties of this bearing steel, namely its elongation, are relatively low; this leads to rather early failures due to surface fatigue, and to socalled pitting, especially if the content of inclusions is high, and if further structure failures, which act as crack originators, are present.

In practice this drawback is partly eliminated by establishing a very considerably limited admissible degree of contamination, by the size of carbide particles, the segregates, and by a number of further factors which substantially influence the price of the material.

SUMMARY OF THE INVENTION It is an object of this invention to provide a steel composition which, to a high degree, covers the requirements of the mechanical properties of anti-friction bearings, particularly as to hardness and strength; these are the factors which to the possible highest degree limit unwelcomed properties, particularly a low elongation. Thus the unfavorable influences of carbide segregates, and of other structural deficiencies are to a high degree limited.

Another object of this invention is to provide a steel composition capable to so-called mechanical heat treatment or ausforming.

DESCRIPTION OF PREFERRED EMBODIMENT The steel composition according to this invention comprises 0.2 to 0.4 percent of eutectoid carbon at a nominal content of 0.6 to 0.9 percent of carbon, together with alloy additions of 1.5 to 2.5 percent of manganese, 1.5 to 2.5 percent of silicon, and 0.7 to 2 percent of chromium, remainder substantially iron, all percentages being by weight, the total of manganese and silicon being at least 3.5%. The difference between the nominal and eutectoidal carbon content is bonded in carbides. This socalled eutectoid carbon is carbon present in the solid solution (in the martensite) after hardening. This carbon content can be determined by an X-ray analysis. The steel comprises in addition carbon bonded in carbides up to a nominal content of 0.6 to 0.9% of C. The overall carbon content in the solid solution and in carbides corresponds to 0.6 to 0.9% and can be determined by commonly used chemical analysis. The above-mentioned content of 0.2 to 0.4% ofeutectoid carbon is, however, decisive for creating a needle shaped martensite structure in the hypereutectoid steel, with the result of improved mechanical properties. The reduction of the eutectoid carbon content to 0.2 to 0.4% is achieved by the above-described alloy additions of Mn, Si, and Cr. Hypereutectoid carbon" is carbon in excess of that in the solid solution, particularly in carbides.

In addition to these alloy additions, the steel can for special purposes contain up to 1 percent of tungsten, or upt to 0.8 percent of vanadium, or up to 0.5 percent of titanium, or up to 0.5 percent of niobium, or up to 0.8 percent of molybdenum, or up to 0.5 percent of titanium, all percentages being by weight. When selecting the amount and kind of urther additions, the condition must be fulfilled that the eutectoid carbon content should not drop below 0.2 percent by weight.

It is possible, in accordance with the newest scientific and research knowledge to obtain a rolling bearing material with approximately 0.2 up to 0.4 percent of carbon, if its chemical composition is chosen so that the eutectoid point is shifted to the lower carbon concentrations. Where the nominal carbon content is higher, for example, approximately 0.6 to 0.9 percent in chromium bearing steel, the eutectoid point of which is not far from 0.8 percent carbon. It is advantageous to bond the excess carbon in the finest carbides. Both requirements are met by admixtures creating carbides, or by admixtures shifting the eutectoid point to a lower carbon concentration. This carbon content in the solid solution guarantees a relatively high interstitial reinforcement.

The superfluous carbon bonded in carbides causes a favorable dispersion reinforcement so that the steel together with some further hardening factors guarantees a high hardness and strength. The carbon content of 0.2 to 0.4 percent in the solid solution gives rise mostly to a needle shaped martensite structure with a dislocated substructure in contrast to prior chromium bearing steel, the structure of which contains plate martensite with inner twins. This different structure has advantageous properties, namely, in the improvement of working properties, and in a higher resistance against contact fatigue.

The material is subsequently subjected to austenitizing at 780 to 820 C for about half an hour with subsequent tempering at 150 to 200 C for about 1 to 1% hour.

An example of an alloy according to this invention has, for instance, a chemical composition of 0.6 to 0.7 carbon, 2 percent of manganese, 2 percent of silicon, 1 percent of chromium, remainder substantially iron, all percentages being by weight. This alloy attains, after quenching from a temperature 780 800 C and after lowtemperature tempering to 150 200 C, the following properties strength limit 200 to 230 kp/mm, proportionality limit to kp/mm, extension 1 to 3.5 percent, hardness 62 up to 65 H In comparison with prior chromium bearing steels, the proposed alloy shows particularly improved shaping properties. The whole carbon content is lower, so

that the danger of creation of carbide liquidation is considerably reduced. The improved shaping properties and the reduction of the occurrence of failures leads to an increase in the lifetime of anti-friction bearings of approximately 40 percent.

The steel which is obtained by the method according to this invention while being subject to stresses as are roller bearings, offers a life time which can be compared only to special steels obtained by the so-called electroslag process, as for instance the steel having the US designation AISI 52 100. This is a very significant effect, as the steel made in accordance with the present invention is manufactured by currently used technology, while the steel remelted by using the electroslag process requires a complicated technology involving high manufacturing costs.

A further substantial advantage of the steel made by the present invention is that it allows the so-called ausforming or mechanical heat treatment. This socalled ausforming consists in heating the steel to the austenitic condition, that is, to 400 to 600 C, in which condition it is mechanical deformed and thereafter quenched.

Although the invention is illustrated and described with reference to a plurality of preferred embodiments thereof, it is to be expressly understood that it is in no way limited to the disclosure of such a plurality of preferred embodiments, but is capable of numerous modifications within the scope of the appended claims.

What is claimed is:

l. A hypereutectoid steel composition particularly for anti-friction bearings consisting essentially of 0.2 to 0.4% carbon present in the martensite after hardening the remainder of the carbon being chemically bonded, the total carbon content being 0.6 to 0.9% together with 1.5 to 2.5% manganese, 1.5 to 2.5% silicon, and 0.7 to 1.2% chromium balance iron, the total of manganese and silicon being at least 3.5%, all percentages being by weight, the steel having been quenched from a temperature of 780 to 820 C, and thereafter having been subjected to tempering at a temperature of 250 C for from I to 2 hours.

2. A hypereutectoid steel composition as in claim 1, comprising in addition at least one of the following alloy additions: up to 1% of tungsten, up to 0.8% of vanadium, up to 5% of titanium, up to 0.5% of niobium, and up to 0.8% of molybdenum, all percentages being by weight. 

1. A HYPEREUTECTOID STEEL COMPOSITION PARTICULARLY FOR ANTIFRICTION BEARINGS CONSISTING ESSENTIALLY OF 0.2 TO 0.4% CARBON PRESENT IN THE MARTENSITE AFTER HARDENING THE REMAINDER OF THE CARBON BEING CHEMICALLY BONDED, THE TOTAL CARBON CONTENT BEING 0.6 TO 0.9% TOGETHER WITH 1.5 TO 2.5% MANGANESE, 1.5 TO 2.5% SILICON, AND 0.7 TO 1.2% CHROMIUM BALANCE IRON, THE TOTAL OF MANGANESE AND SILICON BEING AT LEAST 3.5%, ALL PERCENTAGES BEING BY WEIGHT, THE STEEL HAVING BEEN QUENCHED FROM A TEMPERATURE OF 780* TO 820*C, AND THEREAFTER HAVING BEEN SUBJECTED TO TEMPERING AT A TEMPERATURE OF 150*-250*C FOR FROM 1 TO 2 HOURS.
 2. A hypereutectoid steel composition as in claim 1, comprising in addition at least one of the following alloy additions: up to 1% of tungsten, up to 0.8% of vanadium, up to 5% of titanium, up to 0.5% of niobium, and up to 0.8% of molybdenum, all percentages being by weight. 